Many electronic devices rely on analog-to-digital (A/D) converters to convert an analog signal into a digital signal. Some applications may require different conversion speeds for the same A/D converter. For example, an A/D converter integrated in a digital camera may be required to operate at different speeds depending on the mode of the camera. When the digital camera is operating in high resolution mode, the A/D converter may be required to operate at a higher sample rate than when the digital camera is operating in a low resolution mode. In conventional implementations, the A/D converter is typically designed to operate adequately at the highest required operating speed. In particular, the A/D converter is designed to dissipate enough power to operate correctly at the maximum operating speed.
However, the conventional A/D converter will use the same amount of power when operating at sample rates that are less than the maximum. As a result, the A/D converter dissipates more power than required whenever it operates at less than the maximum operating speed. As a result, the electronic device will dissipate excess power under such conditions. Such is the case for many other circuits, for example, power amplifiers in cellular telephones that operate at different speeds under certain conditions. For example, the cellular phone may operate at a relatively slow speed when in a sleep mode, and operate at a relatively high speed when in an active mode (e.g., when the phone is receiving or initiating a phone call). Accordingly, such circuits will have sub-optimal power consumption when implemented with constant bias currents.
Designing a circuit may require consideration of a number of things that can vary. For example, when a circuit or device is manufactured, there will be some variation in the process. Manufacture of electronic components and/or fabrication of a circuit or device will often cause the operating parameters of the electronic components to deviate from their nominal value, and also deviate from fabrication to fabrication. In addition, the operating parameters of electronic components may vary according to processing conditions of the device. During the design of the circuit, each of the electronic components may be characterized and simulated. To avoid circuit malfunction, the power consumption of the circuit must be sufficient to operate under worst case conditions. Accordingly, the circuit will dissipate the maximum power required under worst case conditions even when conditions require substantially less power.